Plastic fasteners of the general type to which this invention relates have been available in a variety of forms. Each of the various species of such fasteners has been intended to be particularly satisfactory in one type of application while being more or less acceptable for at least a few other applications; but every such fastener has had limitations along with its desirable features. Thus the plastic fastener of U.S. Pat. No. 3,417,438 to T. J. Schuplin was suitable for insertion into a square or rectangular hole in a workpiece but was not well suited for installation in a more conventional round hole. Fasteners suitable for round holes are disclosed in U.S. Pat. Nos. 4,083,289 to Erickson and 3,385,158 to Morin, but the fastener of the Erickson patent was not well suited for installations that were subjected to substantial vibration, and that of the Morin patent, although better suited for withstanding vibration, presented installation problems because its plunger had to be maintained in a critical rotational orientation as it was driven forwardly in the body to its operative position.
Notwithstanding the variety of designs and configurations for plastic fasteners that have been available, there has been a long-standing and heretofore unsatisfied need for a plastic fastener which could be installed in a round hole in a workpiece, could sustain severe vibration, and would be satisfactory for securing together a pair of parts or elements that could have slight motion relative to one another. A typical application for a fastener of the type here under consideration would be for securement of a high density plastic fascia that wraps around the front end portion of an automobile body just above the bumper. The plastic fascia must be permitted to have small motions relative to adjacent body parts, so that it has no tendency to ripple or wrinkle under the influence of road bumps and engine vibrations, but it must nevertheless be fastened securely.
In most prior fasteners of the general type here under consideration, the sleeve-like body of the fastener was so configured that its front end portion was forwardly flared when the plunger was fully inserted into it, so that if the fastener was displaced axially rearwardly relative to a workpiece in which it was installed, its flared front end portion wedged itself more and more snugly against the edge of the hole in which the fastener was seated. This made for a tight connection if the workpiece comprised a relatively hard front panel or part against which the front end portion of the fastener engaged, and such an arrangement was usually very satisfactory for an assembly that was not subject to vibration. Under vibration, however, the fastener and edge portions of the hole could impose very high localized forces upon one another, owing to the wedging relationship between them, and the hole or the fastener could eventually become deformed to the extent of permitting noticeable loosening or total failure of the connection made by the fastener. Of course, such a fastener tended to oppose even slight relative motion between the connection parts and was therefore unsatisfactory for installations where such relative motions had to be permitted.